The present invention relates to a tank adapted to contain liquid and/or compressed gases during storage or transportation and more particularly, to a storage tank which is effective in transporting liquid natural gas in a floating vessel as well as the apparatus required for supporting the tank within the hull of the vessel.
The volume that a gas occupies at normal ambient temperatures is many hundredths of time the volume that it occupies as a liquid. For this reason, many useful gases are stored in their liquid form. Gases can be liquified by either compressing them until their critical pressure is reached or by cooling them until their critical temperature is reached. It has been proven more practical to refrigerate natural gas to its liquid form rather than to compress it. In any event, some petroleum gases are stored and/or transported in a semi-compressed state.
The various forms of containment tanks which are presently being used include those having a spherical, cylindrical or prismatic shape. A recent application of liquid storage tanks is in the marine transportation of liquid natural gas (LNG). However, the storage and marine transportation of liquid natural gas at substantially atmospheric pressure and at very low temperatures, for example, about -260.degree. F., presents many problems. These problems are especially difficult to solve in marine transportation and include, for example, structural stresses due to static and dynamic loading, deflection of the hull, deflection of the tanks, and support requirements adapted to accept temperature contraction and temperature stresses in the hull of the vessel and the tanks. These problems have been overcome in a number of ways including the use of suspended spherical tanks, double walled semi and full membrane tanks and prismatic tanks which are supported by the surrounding insulation which in turn is supported by the hull structure of the vessel. Even though liquid petroleum gases have been transported under pressure in spherical tanks for many years, the early liquid natural gas vessel tanks were constructed as double walled prismatic tanks, that is, tanks having primary and secondary barriers. The forces created in the walls of the tanks were transmitted to the hull structure through the insulation, which had to be of a type capable of carrying a load at low temperatures.
The above described tank systems with the exception of the suspended spheres, require considerable manpower to install. Also testing of the installation in its ability to absorb and transmit stresses from the tank to the hull of the vessel, is difficult. Self supporting independent tanks overcome the testing problem and their insulation is much simpler. Although spherical tanks have been used as an effective type of self supporting independent tank, and a number of methods for supporting them in the hull of the vessel have been proposed, the sphere is an inefficient user of space in a vessel.
Accordingly, an object of the present invention is to provide a self supporting, independent liquid or compressed gas containment tank which has a unique shape which contributes to the overall strength of the tank.
Another object of the present invention is to provide an improved self supporting storage tank which, because of its geometric shape lends itself to the accurate determination of stresses to which the tank will be subjected and thus, can be readily adapted to meet the various classification and regulatory body requirements of a single walled containment system for either land storage or the marine transportation of refrigerated liquid natural gas.
A further object of the present invention is to provide an improved storage tank which, because of its geometric shape, is capable of storing the same quantity of liquid and/or gas as the prior art spherical tanks even though it possesses smaller overall dimensions.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Pursuant to the present invention, the above-mentioned disadvantages may be eliminated and a substantially improved self supporting storage tank adaptable for land storage and vessel storage in connection with marine transportation has been developed wherein through the use of a plurality of cylindrical and spherical shapes which are combined in an effective manner, a tank of improved strength and capacity is produced. The storage tank of the present invention is a self-supporting, independent liquid or compressed gas containment tank, the boundaries of which are made up of intersecting cylinders, parts of spheres and cylindrical caps.
The present invention is based on the knowledge that both the sphere and the cylinder are efficient forms for pressure vessels. However, as the volume of the tank increases, it becomes expensive and eventually impractical to construct the tanks as individual spheres or cylinders due to the required thickness of the shell. The present invention overcomes this problem by combining cylinders and spheres into a unique relationship and using the resulting shape as the boundaries for the tank. Because the tank is constructed solely from intersecting portions of cylinders and spheres, the stresses can be readily determined using the available structural analysis theory for thin shells. In order to obtain a better volumetric efficiency, that is, by dividing the tank volume by the volume of the circumscribing cube, the space inside the external boundaries of the tank but outside the cylinders is made part of the tank volume by fitting six cylindrical closing caps in the square holes in the six sides of the tank and removing a 90.degree. arc length of the cylinder within the boundaries of the tank. This deletion of a part of the cylinders would normally result in unbalanced loads in the individual cylinders. However, the manner in which the cylinders and the cylindrical closing caps are connected together enables these loads to be balanced and provides a stable structure. Due to the shape of the tank, being made up of rotationally symmetric plates, the design can be fully analyzed by various structural analysis methods such as limit analysis, finite element analysis and fracture mechanics, thus enabling the containment system to be classed as a "leak before catastrophic failure" type system. As it is a single walled vessel of relatively simple structural design, including the support system, the completed tank can be tested by non-destructive methods including hydrostatic or compressed air pressure tests. This in turn enables the tank to be readily accepted by the various regulatory agencies.
The storage tank of the present invention possesses inherent strength and good volumetric efficiency because of its unique shape. The weight of the tank and its contents is advantageously distributed to four corner pedestals disposed at the bottom of the tank. The tank can be restrained from lateral movement either at the corner pedestal plates or at a higher location on the tank depending on the intended use of the tank. Because the tanks are basically a combination of cylindrical and spherical shapes, they function as effective pressure vessels which can be provided with internal stiffeners or merely utilized as unstiffened shells. Also, because of their improved strength and volume efficiency, if the tanks of the present invention are used in a liquid natural gas tanker in place of the presently utilized spherical tanks, the length, width and depth of the tanker can be reduced by approximately 8%, 10% and 15%, respectively. With size reductions in mind, it is readily apparent that the expense of construction of the smaller size tankers can be reduced by about 10-20%. Also, there is the additional savings in the construction of the smaller size tanks. For example, using the tanks of the present invention in a 125,000 cubic meter liquid natural gas tanker, the length, width, and depth of said tanker can be reduced by 75 feet, 15 feet and 15 feet, respectively when compared with an equivalent spherical tank vessel. Thus, by utilizing the tanks of the present invention, the ship portion of the total cost can be reduced by up to about 20% and the cost of the containment tanks can be reduced also by a significant amount. Of particular importance is the fact that no machining of large parts is required and because the diameter of the cylinder is smaller than about one-half of the overall dimensions of the tank, the tank structure is thinner than spherical tanks of equal volume. This feature, of course, reduces material, welding, handling and shaping problems. The tank lends itself to construction by existing structural fabricators without the need for special machines. The tank can also be installed in a ship either in pieces or as a complete tank.
The tank can be designed for use as a fixed land liquid or compressed gas storage tank and in addition, the unique shape of said tank comprising parts of cylinders and spheres makes it suitable for a number of uses where the vessel is subjected to either internal or external pressures.